The invention relates to a fluorescent lamp having a sealed discharge vessel with an inside surface provided with a layer of luminescent material which is a mixture of phosphors which luminesce in different wavelength ranges to produce white light.
Fluorescent lamps typically have a transparent glass envelope enclosing a sealed discharge space containing an inert gas and mercury vapor. When subjected to a current provided by electrodes, the mercury ionizes to produce radiation having primary wavelengths of 185 nm and 254 nm. This ultraviolet radiation, in turn, excites phosphors on the inside surface of the envelope to produce visible light which is emitted through the glass. The phosphors are typically chosen to emit light in each of the three primary colors and are therefore referred to as red, green, and blue phosphors.
EP 0 067030 discloses a fluorescent lamp having a glass tube coated with a mixture of phosphors including at least one red phosphor, at least one green phosphor, and at least one blue phosphor. The red phosphor may include yttrium oxide activated by europium (YOX), which is usually the sole red emitter. The green phosphor may include lanthanum phosphate activated by cerium and terbium (LAP). The blue phosphor may include barium magnesium aluminate activated by europium (BAM) and/or strontium calcium halophosphate activated by europium (SCAP). EP 067030 recognizes that wall loading (W/m.sup.2) increases as the diameter of the envelope decreases. The intensity of ultraviolet radiation having wavelengths of 185 nm and 254 nm also increases. However the 185 nm wavelength damages conventional phosphors, so it is not possible to achieve the desired lumen maintenance. The problem is addressed by increasing the relative amount of red phosphor, which absorbs the 185 nm radiation and prevents deterioration of the blue and green phosphors. This improves luminous flux and maintenance factor but skews the chromaticity toward red. Further, some blue and green phosphors have a high consumption of mercury, making them unsuitable for use in low mercury lamps regardless of wall loading.
The apparent color of a light source is described in terms of color temperature, which is the temperature of a black body that emits radiation of about the same chromaticity as the radiation considered. A light source having a color temperature of 3000.degree. K. therefore has a larger red component than a light source having a color temperature of 4100.degree. K. The color temperature of a phosphor mixture can be varied by changing the ratio of the phosphors.
Color quality is further described in terms of color rendering, and more particularly color rendering index (CRI or R.sub.a), which is a measure of the degree to which the psycho-physical colors of objects illuminated by a light source conform to those of a reference illuminant for specified conditions.
CRI is in effect a measure of how well the spectral distribution of a light source compares with that of an incandescent (blackbody) source, which has a Planckian distribution between the infrared (over 700 nm) and the ultraviolet (under 400 nm). The discrete spectra which characterize phosphor mixtures will yield good color rendering of objects whose colors match the spectral peaks, but not as good of objects whose colors lie between the spectral peaks.